Method and apparatus for registration control during processing of a workpiece, particularly during producing images on substrates in preparing printed circuit boards

ABSTRACT

A method and apparatus for controlling a processing machine to perform a processing operation on a workpiece by (a) determining the nominal locations of at least two sensible reference marks on the workpiece in terms of the coordinates of the workpiece; (b) loading the workpiece on the processing machine; (c) sensing, and measuring the actual locations of, the reference marks on the workpiece when so mounted, in terms of the coordinates of the processing machine; (d) determining at least one geometrical transformation needed to transform the workpiece coordinates of the nominal locations of the reference marks to the processing machine coordinates of the actual locations of the reference marks; (e) and controlling the processing machine in accordance with the determined geometrical transformation.

“This application is a division of patent application Ser. No.09/241,433, filed Feb. 2, 1999” now U.S. Pat. No. 6,205,364.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for registrationcontrol during the processing of workpieces. The invention isparticularly useful in registration control during the production ofimages or other process performed with respect to printed circuit boards(PCBs) or other substrates, and is therefore described below withrespect to such an application.

Registration control is important in many processing operations to beperformed on workpieces. It is particularly important in processingoperations requiring the precise matching of a film or an electronicimage (i.e., the eletronic representation of an image) to a givensubstrate. Examples of the latter processing operations include contactprinting of patterns in multi-layer PCBs, manufacturing of integratedcircuits in microelectronics, and plotting of pre-press tooling panels.For example, manufacturing multi-layer PCBs involves the fabrication,and then the stacking, of up to 20 or 30 layers, in which each layer(commonly referred to as an inner layer) has its own previouslygenerated conductor or other (e.g., through holes) pattern.

Manufacturing such multi-layer PCBs encounters a number of registrationproblems, particularly the following: (1) registration of the image ofeach layer with respect to those in the other layers; (2) registrationof an image on one side of a layer with respect to an image on the otherside of the layer; (3) registration of the image plotted on each side ofthe layer with respect to other tasks involved in processing the layeror the board, such as outer-layer processes, drilling holes, etc.; and(4) registration of the image on each side of the layer with respect toexisting pre-drilled holes through the layer. These registrationproblems become increasingly more difficult to overcome as thecomponent-density of the boards, wafers, or other workpieces increases,the number of layers increases, and/or the size of the workpiecedecreases.

Various techniques for overcoming these types of registration problemsare described in U.S. Pat. Nos. 4,829,375, 5,136,948, 5,164,742,5,274,394, 5,403,684, 5,453,777, 5,459,941, 5,500,801, 5,548,372, and5,699,742.

Most of the known techniques generally involve reorienting the substratewith respect to the machine, and then plotting the image from the imagefile. However, such known techniques have a number of drawbacks,particularly when plotting on PCBs or other substrates under conditionsproducing variations in thickness in the layer, or variations inthickness or in length among a series of layers. For instance, when asubstrate having significant thickness is mounted on a cylindrical drumwhich is rotated with respect to the printing or plotting elements, theouter surface of the substrate is under tension thereby increasing itslength, whereas the inner surface of the substrate is under compressionthereby decreasing its length. An image plotted from an image file onthe tensioned outer surface of the loaded layer will shrink when thelayer is unloaded; whereas an image plotted on the compressed innersurface will expand when the layer is unloaded. These effects producescaling changes between the plotted image relative to the image file.These scaling changes, which depend on the thickness of the layer andthe loading conditions, introduce registration problems when printingimages from individual image files on a plurality of overlying layers oron the opposite faces of the layers.

Another registration problem is encountered when imaging two sides of alayer, one after the other in the plotting machines which can not plotthe two sides of the layer simultaneously. This problem is present withrespect to both rotary-drum and flat-bed imaging or plotting machines.

A still further registration problem is introduced by dimensionalchanges of the substrate during processing. For example, temperaturevariations during processing induce thermal expansion or contraction ofthe substrate. Thus, thermal changes during the processing of differentlayers produce dimensional changes which generate registration errorswhen the layers are stacked to produce the multiple-layer PCB.Dimensional changes are also introduced by mechanical deformation of thelayers during processing (e.g., mechanical pre-cleaning), deformationdue to stress release (e.g., due to heating in the resist-coating stepor due to laminate deformation after copper etching), etc. Theseregistration problems become much more difficult to overcome when thedimensional changes are local and therefore not correctable by a globalcompensation.

OBJECT AND BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and apparatusfor controlling a processing operation to be performed on a workpiece,particularly an imaging or printing operation to be performed on a PCBinner layer or other substrate, which method and apparatus particularlyaddress some or all of the foregoing registration problems.

According to one aspect of the present invention, there is provided amethod of controlling a processing machine to perform a processingoperation on a workpiece, comprising: (a) determining the nominallocations of at least two sensible reference marks on the workpiece interms of the coordinates of the workpiece; (b) loading the workpiece onthe processing machine; (c) sensing, and measuring the actual locationsof, the reference marks on the workpiece, when so mounted, in terms ofthe coordinates of the processing machine; (d) determining at least onegeometrical transformation needed to transform the workpiece coordinatesof the nominal locations of the reference marks to the processingmachine coordinates of the actual locations of the reference marks; and(e) controlling the processing machine in accordance with the determinedgeometrical transformation.

It will thus be seen that the method as set forth above does notre-orient the workpiece with respect to the processing machine, as insome of the prior art techniques describe in some of the above-citedpatents, but rather changes the workpiece coordinates to the processingmachine coordinates after the workpiece has been mounted on theprocessing machine, and thereby compensates for changes in theworkpiece, such as changes in the length or in the mounting position ofthe workpiece, when mounted on the processing machine.

According to further features in the preferred embodiments of theinvention described below, the actual location or each of the workpiecereference marks is determined in step (c) by; fixing a sensing device tothe processing machine for sensing the workpiece reference marks, with areference point of the sensing device being at a known location withrespect to a reference point of the machine; and vectorially adding theline vector from the machine reference point to the sensing devicereference point, and the line vector from the sensing device referencepoint to the respective workpiece reference point.

More particularly, in the described preferred embodiment, the actuallocation of each of the workpiece reference marks is measured in step(c) by: (i) fixing a camera to the processing machine such that areference point of the camera field of view is at a known location withrespect to a reference point of the machine; (ii) actuating the camerato view a portion of the workpiece which includes the respectiveworkpiece reference mark; (iii) measuring the location of a referencepoint on the respective workpiece reference mark in the camera field ofview, relative to the camera field reference point; (iv) and vectoriallyadding the line vector from the machine reference point to the camerafield reference point, and the line vector from the camera fieldreference point to the respective workpiece reference point to therebydetermine the location of the respective workpiece reference pointrelative to the machine reference point.

According to another aspect of the invention, there is provided a methodof controlling the plotting head of an imaging machine for producing animage on a substrate according to an image file, comprising: (a)determining the nominal locations of at least two sensible referencemarks on the substrate in terms of the coordinates of the substrate; (b)loading the substrate on the imaging machine having a plotting head; (c)sensing, and measuring the actual locations of, the reference marks onthe mounted substrate in terms of the coordinates of the imagingmachine; (d) determining at least one geometrical transformation neededto transform the substrate coordinates of the nominal locations of thereference marks to the imaging machine coordinates of the sensedlocations of the reference marks; (e) and controlling the plotting headto produce an image on the substrate in accordance with the determinedgeometrical transformation.

According to yet another aspect of the present invention, there isprovided a method of processing a substrate comprising: (a) performing afirst processing operation on the substrate; (b) mounting the substrateon a processing machine to perform a second processing operationthereon; (c) sensing the actual locations of preselected features on thesubstrate produced by the first processing operation; (d) andcontrolling the second processing operation in accordance with thesensed features.

According to further features of this aspect of the invention in thepreferred embodiment described below, steps (c) and (d) are performedby: (i) determining, before step (b), the nominal locations of at leastone reference feature on the substrate in terms of the coordinate of thesubstrate; (ii) after step (b), sensing, and measuring the location of,the reference feature in terms of the coordinates of the machine toperform the second operation on the substrate; (iii) determining atleast one geometrical transformation needed to transform the substratecoordinates of the nominal location of the reference feature to themachine coordinates of the sensed location of the reference feature; and(iv) controlling the second processing operation in accordance with thedetermined geometrical transformation.

As will be described more particularly below, the second processingoperation is thus controlled by the actual location of features on thesubstrate produced during the first processing operation, and therebycorrects any errors produced during the first operation performed on thesubstrate.

For example, the first operation may be drilling holes in accordancewith a drilling file in a PCB inner layer; and the second processingoperation may be printing a conductor image from a conductor patternfile, in which conductor pads are to coincide with drilled holes toenable the proper connections to be made to the conductor patterns inthe various inner layers, e.g., by passing conductor pins through theholes. Thus, the printing operation is controlled by adjustment of theprinting elements according to the actual location of some or all of thedrilled holes in a way to bring the conductor pads to their properpositions relative to the drilled holes, thus eliminatingmisregistration between holes and pads. The same technique may beapplied to correct misregistration between holes and pads where the padsare produced during the first processing operation, and the holes are tobe produced during the second processing operation based on the sensedactual locations of the pads.

According to a still further aspect of the present invention, there isprovided a method of controlling an imaging operation performed by aplotting head on the surface of a substrate having thickness,comprising: continuously measuring the thickness of the substrate; andcontinuously controlling the plotting head and the electronic image tocompensate for variations in the thickness of the substrate.

According to a still further aspect of the invention, there is providedapparatus for controlling a processing operation on a workpiece carryingsensible reference marks at nominal locations on the workpiece in termsof the coordinates of the workpiece, comprising: a mounting device formounting the workpiece to the apparatus; a processing head movable withrespect to a workpiece when mounted on the mounting device; a sensingdevice carried by the processing head so as to move therewith withrespect to a mounted workpiece, the sensing device being capable ofsensing the workpiece reference marks and having a reference point whichis at a known location with respect to a reference point on theprocessing head; and a data processor system processing the output ofthe sensing device for: (a) determining the locations of the referencemarks on the workpiece in terms of the coordinates of the processinghead; (b) determining the geometrical transformations needed totransform the workpiece coordinates of the nominal locations of thereference marks to the processing head coordinates of the sensedlocations of the reference marks; and (c) controlling the processingoperation in accordance with at least one of the geometricaltransformations.

Further features and advantages of the invention will be apparent fromthe description below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a top view schematically illustrating one form of processingapparatus, namely an imaging or plotting machine, constructed inaccordance with the present invention;

FIG. 2 is a side view of the apparatus of FIG. 1;

FIG. 3 illustrates the area scanning pattern in the apparatus of FIGS. 1and 2;

FIG. 4 schematically illustrates the electrical system for controllingthe apparatus of FIGS. 1 and 2;

FIG. 5 is a block diagram illustrating the data flow in the apparatus ofFIG. 4;

FIG. 6 illustrates one form of a device for producing reference marks onthe substrate (workpiece) before being applied to the apparatus of FIGS.1-5;

FIGS. 7 and 8 are end and front views, respectively, illustrating thereference marks after application to the substrate by the device of FIG.6;

FIG. 9 illustrates the method applied to a flat-bed imaging or printingmachine for reducing or eliminating registration errors in accordancewith the present invention;

FIG. 10 is a flow chart illustrating one manner of using the describedapparatus for reducing or eliminating registration errors;

FIG. 11 is a flow chart more particularly illustrating one of the stepsin the flow chart of FIG. 9;

FIG. 12 is a diagram helpful in understanding the flow charts of FIGS.10 and 11;

FIG. 13 is a flow chart illustrating another method of reducing oreliminating registration errors in accordance with another aspect of thepresent invention;

FIG. 14 is a flow chart illustrating one implementation of the method ofFIG. 13;

FIG. 15 is a flow chart illustrating an alternative implementation ofthe method of FIG. 13; and

FIG. 16 is a diagram illustrating another manner for eliminating orreducing registration problems arising from variations in the thicknessof the substrate.

DESCRIPTION OF PREFERRED EMBODIMENTS

As will be described more particularly below, the invention is generallyuseful for controlling a processing operation to be performed on aworkpiece in order to eliminate or reduce certain registration errors,particularly those arising when the workpiece is mounted on theprocessing machine. The invention is specially useful for reducing oreliminating registration errors when plotting or printing an image on asubstrate, such as an inner layer of a printed circuit board, mounted onthe external (or internal) surface of a rotary-drum or flat-bed imagingmachine; and therefore the invention is described below with respect tothis particular application.

FIGS. 1-5 illustrate one form of imaging machine of this type, namely alaser direct imaging (LDI) machine which plots (or prints) separateimages from separate files on both sides of an inner layer of a PCB(printed circuit board), similar to the pre-press imaging apparatusmanufactured by Creo Products Inc, of Canada. Such machines can handleone layer at a time, or two layers simultaneously. The one or two layersare manually or automatically loaded on the machine with one side facingupwardly, whereupon the machine plots the file suitable for that side onthe one or two layers. The layers are then manually or automaticallyinverted such that the other side faces upwardly, whereupon the machineplots the file suitable for the second side. After both sides have thusbeen printed, the layers are unloaded.

The method and apparatus described below enable achieving the followingtwo major objectives: (1) to plot the image on each side of the layer ina correct geometry such that the resulting image will resemble the fileupon which it was created in respect of geometrical shape and scale,irrespective of variation in the thickness or the length of the layer,or the alignment of the layer with respect to the machine; and (2) toplot the images on the two sides of the layer such that they willregister with each other.

FIGS. 1 and 2 illustrate two such layers 2 a, 2 b, mounted in anyconventional manner on a cylindrical drum 4 rotatable about a rotaryaxis 5. Each layer 2 a, 2 b, has a resist-coated outer surface to beexposed to the laser beams produced by a linear array of lasers 6carried by a plotting or printing exposure head 7. Each laser defines apixel of the image to be printed on the layers 2 a, 2 b, and is on-offcontrolled according to the respective image file.

The exposure head 7 is mounted on a flat carriage 8 which moves alongtracks 9, by rotating screw 10, parallel to the rotary axis 5 of thedrum 4. The lasers 6 are arranged in a linear array also parallel to thedrum rotary axis 5, such that the rotation of the drum, and the linearmovement of the exposure head 7, cause the laser beams to scan thecomplete area of each layer 2 a, 2 b, in the form of parallel inclinedbands as shown at 4 a in FIG. 3.

A sensing device, in the form of an electronic camera 11, is fixed tothe exposure head 7 so that it moves with the exposure head. Camera 11has a field of view which covers only a relatively small portion of thesurface of the layers 2 a, 2 b in order to provide high resolution in arelatively compact camera. The camera is fixed to the exposure head 7such that a reference point of the camera field of view is at a knownlocation with respect to a reference point of the exposure head 7, andthereby of the lasers 6 producing the laser writing beams. As will bedescribed more particularly below, the camera is used to snap featureson the panel in order to sense reference marks thereon, and thereby todetermine the locations of such reference marks in terms of the machineexposure head coordinates.

Exposure head 7 further carries an autofocus device 12. This devicemeasures the distance between the exposure head and the outer surface ofthe layer, by means well known in the art, in order to keep the printinglaser beams in focus with the layer outer surface. However, as will alsobe described more particularly below, autofocus device 12 is also used,according to another aspect of the present invention, for continuouslymeasuring the thickness of the layer 2 a, 2 b, and for continuouslycontrolling the lasers 6 to compensate for geometrical distortionsresulting from variations in layer thickness.

As shown in FIG. 4, the electrical system includes two main processingunits: a workstation (WS) processor 15 located in the workstationoutside of the imager; and an imager processor 16 located on the imager.The two processors communicate with each other via a two-directionalpath 17.

WS processor 15 is the main control unit. As shown in FIGS. 4 and 5, WSprocessor 15 receives inputs from the image file 18 and the userinterface 19, and controls the lasers 6 and the camera 11 carried by theexposure head 7 in accordance with these inputs. A frame grabber 20within the WS processor 15 grabs the video signal frame from camera 11received via line 21, and converts it to a graphic file.

As will be described more particularly below, the WS processor 15identifies certain features in the grabbed frame, and calculates thegeometrical corrections that are to be applied on the electronic image(i.e., the electronic representation of the image) from the image file18. These corrections are sent to the imager processor 16, whichperforms the appropriate electronic and data manipulations on theelectronic image to correct for misregistration in the layers 2 a, 2 b.

The imager processor 16 includes a data buffer 22 which receives thedata from the WS processor 15. A graphic file of a given format isconverted in the WS processor 15 to a bit-map file, and is sent througha dedicated path 23 to the data buffer 22 of the imager processor 16.Data is sent from a suitable location in data buffer 22 via path 24 tothe exposure head 7 to control the plotting lasers 6.

As shown particularly in FIG. 5, the workstation (WS) operations aregoverned by the LDI (Laser Direct Imaging) software within the WSprocessor 15; and the actions of the lasers 6 carried by the exposurehead 7 are governed by the imager processor 16, which processor alsoreceives feedback from the exposure head.

According to one aspect of the present invention, registration problemsarising by changes in thickness and in length of the layer when mountedon the imager (e.g., rotary drum 4) are eliminated or reduced byconverting the workpiece coordinates of the image file 18 to the machinecoordinates of the exposure head 7 and the drum 4 of the plottingmachine. Briefly, this is done in the manner described in the flow chartof FIG. 10, by controlling the plotting elements (lasers 6) for plottingthe image on the outer surfaces of the layers 2 a, 2 b in the followingsteps:

(a) determining the nominal location of at least two sensible referencemarks on each substrate (layers 2 a, 2 b) in terms of the coordinates ofthe substrate (block 40, FIG. 10);

(b) loading the substrate (layers 2 a, 2 b) on the rotatable drum 4 ofthe imaging machine (block 41);

(c) sensing the reference marks and measuring their locations in termsof the coordinates of the imaging machine (block 42);

(d) determining the geometrical transformations needed to transform thesubstrate coordinates of the nominal locations of the reference marks tothe imaging machine coordinates of the sensed locations of the referencemarks (block 43);

(e) and controlling the array of lasers when plotting on the substrate(layers 2 a, 2 b) in accordance with the at least one geometricaltransformation (block 44).

Such a method is to be contrasted to presently known aligning methodsdescribed in some of the above-cited prior patents, which known methodsgenerally use features on the substrate (pre-punched holes, fiducialmarks, etc.) to physically orient or re-orient the substrate to fit themachine coordinates. In the present invention, however, the substrate isnot physically re-oriented, but rather the data flow of the datarepresenting the electronic image is controlled to fit the machinecoordinates.

FIGS. 6-8 illustrate one manner of determining the nominal locations ofat least two sensible reference marks on the substrate in terms of thecoordinates of the substrate. This may be done by a marking device,generally designated 30, including a table 31 having supporting surface(e.g., defined by a plurality of rollers 32 or by a sildable surface)for receiving the substrate 2 and for enabling it to move along thetable. The table further includes a stop, schematically indicated at 33,for stopping the substrate at a predetermined position on the table. Amarking implement carries two marking elements 34, 35 on each side atpredetermined locations with respect to stop 33, so as to applyreference marks to both sides of the substrate 2 when in thepredetermined position determined by stop 33.

All four reference marks are applied simultaneously, two (34 a, 35 a) onone side (2 a, FIG. 7) of the substrate, and the other two (34 b, 35 b)on the other side 2 b. This assures extreme accuracy of the positioningof the reference marks relative to each other, rather than relative tothe substrate edges, since these reference marks are used forregistration purposes rather than the substrate edges.

Table 31 of marking device 30 is preferably made of a material havingthermal expansion parameters close to those of the substrate (layers 2a, 2 b) in order to minimize temperature effects and to provide highrepeatability.

FIG. 9 illustrates a second form of imaging machine of the typeincluding a flat-bed 104 for receiving the substrate layer 102 on whichan image is to be plotted by one or more lasers of a laser unit 106acarried by a plotting or exposure head 107. In this example, theflat-bed 104 is driven by a drive 110 along the Y-axis, and the laserbeam 106 is deflected along the X-axis by a rotatable polygon and mirrorassembly 108. The exposure head 107 also carries a camera 111 and anautofocus device 112, corresponding to camera 11 and autofocus device 12in FIGS. 1 and 2.

The machine illustrated in FIG. 9 is otherwise the same as therotary-drum machine described above, in which the laser 106 is to becontrolled to plot an image on both faces of layer 102. Accordingly,layer 102 would be marked with the two reference marks 134 a, 135 a onone face, with the other two reference marks (not shown) on the oppositeface, corresponding to the four reference marks applied to layer 2 asillustrated in FIGS. 7 and 8.

OPERATION

The flow charts of FIGS. 10 and 11 illustrate one manner of operatingeither of the above-described machines for purposes of reducing oreliminating misregistration of images, particularly those arising fromvariations in the thickness, length or loading orientation of thesubstrate layer (2 or 102) when loaded on the rotary-drum 4 of theimaging machine illustrated in FIGS. 1-5, or on the flat-bed 104 of theimaging machine illustrated in FIG. 9.

Before the substrate is loaded onto the machine, a determination is madeof the locations in workpiece coordinates of at least two referencemarks on the substrate. This may be done by using the marking deviceillustrated in FIGS. 6-8 to apply the two reference marks (e.g., 34 a,35 a) on one face of the substrate, and two other reference marks (e.g.,34 b, 35 b) on the other face. The two reference marks may be ofcircular configuration each defined by a reference point therein (e.g.,the center of the circle) whose position is precisely known in thesubstrate coordinates, e.g., in the coordinates of the image file.

Rather than specially making these reference marks, existing features onthe substrate having precisely known locations in the substratecoordinates may also be used as reference marks for this purpose. Forexample, the substrate coordinates of such previously existing marks maybe their coordinates in a file created in a previous processing step.

The substrate is loaded onto the drum 4 of the machine illustrated inFIGS. 1 and 2 (block 41, FIG. 10), or onto the flat-bed 104 of themachine illustrated in FIG. 9, with one side facing outwardly forplotting thereon. Reference marks 34 a, 35 a (or 134 a, 135 a) will thusbe exposed.

The next step is to sense each reference mark on the substrate and todetermine its location in machine coordinates (block 42). This is doneby using the camera 11 (or camera 111) according to the proceduredescribed in the flow chart of FIG. 11.

Thus, as shown in FIGS. 11 and 12, camera 11 (or camera 111) is fixed tothe exposure head 7 (or 107) of the plotting machine such that a camerareference point (RP_(C), FIG. 12), e.g., the center point of the camerafield of view, (FO_(V)) is at a known location with respect to a machinereference point (RP_(M)) of the exposure head 7 (or 107) on which thecamera is fixed.

The exposure head 7 (or 107), and the camera 11 (or 111) fixed to it,are then moved to a “snap” position of the camera, wherein one of thetwo substrate (workpiece) reference points (RP_(S)), e.g., 34 a, iswithin the field of view of the camera. This position is snapped byframe grabber 20, and the frame is converted to a graphic file (block 42b). Processor 15 then measures the location of the substrate (workpiece)reference point RP_(S) relative to the camera field reference pointRP_(C) (block 42 c), and then vectorially adds the line vector from themachine reference point RP_(M) to the workpiece reference point RP_(s),and the line vector from the camera field reference point RP_(C) to theworkpiece reference point RP_(S) (block 42 d). This vectorial additiondetermines the exact location of the reference mark (e.g., 34 a) inmachine coordinates.

In this example, the reference mark 34 is shown as of a circularconfiguration, with the reference point thereof being in the center; itwill be appreciated, however, that other configurations of referencemarks may be used (e.g., square configuration), and that other pointsthereof (e.g., a corner) could be considered as the reference point ofthe reference mark.

It will be seen, from FIG. 12, that vector line V₁, connecting themachine reference point RP_(M) with the camera field reference pointRP_(C), is known beforehand; that vector line V₂, connecting the camerafield reference point RP_(C) with the workpiece reference point RP_(S),is measured by operation 42 c of FIG. 11; and that vector line V₃,connecting the machine reference point RP_(M) to the workpiece referencepoint RP_(S), is determined by operation 42 d. Accordingly, operation 42d determines the precise position of the reference point RP_(S) of thereference mark 34 in terms of the machine coordinates.

The operations illustrated in the flow chart of FIG. 11 are performedseparately for each of the two reference marks 34 a, 35 a, such that thelocation of each reference mark is determined in terms of the machinecoordinates.

The WS processor 15 then determines the geometrical transformationsrequired to transform the workpiece coordinates of the two referencemarks 34, 35, to machine coordinates (block 43, FIG. 10). Preferably,these geometrical transformations include scale transformation, rotationtransformation, X-axis shift transformation, and Y-axis shifttransformation. Following are the calculations of the four geometricaltransformations performed in the WS processor, and the order in whichthey are performed,to transform the reference mark coordinates from thesubstrate (nominal) to the machine (actual) coordinates.

(1) S=scale, stretching or shrinking the nominal vector;

(2) R=rotation angle in which the nominal vector is rotated;

(3) DX=x-axis (drum circumference) shift of the nominal vector;

(4) DY=y-axis (carriage) shift.

These four numbers are calculated in the WS processor 15 and are sent tothe machine processor 16.

The nominal (substrate) coordinates of the two reference marks are: (x₁^(N),y₁ ^(N));(x₂ ^(N),y₂ ^(N)). The actual coordinates are:(x₁,y₁);(x₂,y₂).

After each of the two transformation is calculated, the transformationis applied on the nominal positions to yield transformed coordinatesneeded for calculating the next transformation.$\underset{\_}{(1)\quad {Scaling}\quad \text{:}}$${S = \frac{\overset{\rightarrow}{r}}{{\overset{\rightarrow}{r}}_{N}}},{{where}\quad \text{:}}$$\begin{matrix}{{{\overset{\rightarrow}{r}}_{N}} = \sqrt{\left( {x_{2}^{N} - x_{1}^{N}} \right)^{2} + \left( {y_{2}^{N} - y_{1}^{N}} \right)^{2}}} \\{{\overset{\rightarrow}{r}} = {\sqrt{\left( {x_{2} - x_{1}} \right)^{2} + \left( {y_{2} - y_{1}} \right)^{2}}.}}\end{matrix},$

Then, the new nominal coordinates: x₁^(N1) = S ⋅ x₁^(N);y₁^(N1) = S ⋅ y₁^(N); x₂^(N1) = S ⋅ x₂^(N); y₂^(N1) = S ⋅ y₂^(N)

$\underset{\_}{(2)\quad {{Rotation}:}}$${R = {\cos^{- 1}\left\lbrack \frac{{\left( {x_{2} - x_{1}} \right)\left( {x_{2}^{N1} - x_{1}^{N1}} \right)} + {\left( {y_{2} - y_{1}} \right)\left( {y_{2}^{N1} - y_{1}^{N1}} \right)}}{\sqrt{\left\lbrack {\left( {x_{2} - x_{1}} \right)^{2} + \left( {y_{2} - y_{1}} \right)^{2}} \right\rbrack \cdot \left\lbrack {\left( {x_{2}^{N1} - x_{1}^{N1}} \right)^{2} + \left( {y_{2}^{N1} - y_{1}^{N1}} \right)^{2}} \right\rbrack}} \right\rbrack}},$

and then: $\begin{matrix}{x_{1}^{N2} = {{x_{1}^{N1} \cdot {\cos (R)}} - {y_{1}^{N1} \cdot {\sin (R)}}}} \\{y_{1}^{N2} = {{x_{1}^{N1} \cdot {\sin (R)}} + {y_{1}^{N1} \cdot {\cos (R)}}}} \\{x_{2}^{N2} = {{x_{2}^{N1} \cdot {\cos (R)}} - {y_{2}^{N1} \cdot {\sin (R)}}}} \\{y_{2}^{N2} = {{x_{2}^{N1} \cdot {\sin (R)}} + {y_{2}^{N1} \cdot {\cos (R)}}}}\end{matrix}$ $\underset{\_}{(3)\quad {Shift}\quad X\text{:}}$${DX} = {\frac{x_{1} + x_{2}}{2} - \frac{x_{1}^{N2} + x_{2}^{N2}}{2}}$$\underset{\_}{(4)\quad {Shift}\quad Y\text{:}}$${DY} = {\frac{y_{1} + y_{2}}{2} - \frac{y_{1}^{N2} + y_{2}^{N2}}{2}}$

The numbers (S,R,DX,DY) are sent to the imager processor 16, whichapplies them on the electronic image as well known in the art.

It is not necessary to determine and use all the foregoing geometricaltransformations, as in some applications it may be sufficient orappropriate to determine and/or use only one (e.g., scaling), two, orthree of such transformations.

The processing operation (e.g., imaging, plotting, printing) is nowcontrolled by the image processor 16 in accordance with the determinedgeometrical transformation or transformations, as indicated by block 44of FIG. 10. By manipulating the electronics and data flow, the imagerprocessor 16 is able to apply one or more of the desired geometricaltransformations (e.g., shift, scaling and rotation) on the imaged fileduring imaging, by means well known and commonly used in the existingcommercial equipment, such as in the LDI (Laser Direct Imaging) systemfor imaging printing-press plates produced by Creo Products Inc. ofCanada. For example, image scaling may be controlled by changing thedata transfer rate to the substrate, and/or by changing the relativevelocity between the drum or flat-bed and the writing beams.

The geometrical transformations applied by the imager processor 16 onthe electronic image may have any desired algebraic relation to thegeometrical transformations determined by the WS processor 15.Preferably, in order to achieve correct image geometry and scale, andlater on side to side registration, the same transformations totransform the workpiece coordinates of the reference marks to themachine coordinates are applied on the electronic image during thewriting or plotting of the image by the laser printing elements 6 on therespective side of the substrate.

In the example illustrated in FIGS. 7 and 8, images are also to beprinted on the opposite side of the substrate, which is the reason whyreference marks 34 b, 35 b, are also applied to the opposite side of thesubstrate. Accordingly, after one side has been correctly plotted in themanner described above, the substrate is turned over, and theabove-described procedure is repeated for plotting on the opposite side,such that the images plotted on both sides are properly registered andscaled with respect to the reference marks, and thereby with respect toeach other, and not to the edges of the substrate.

It will also be appreciated that if two (or more) substrates are handledat the same time, as shown in FIG. 2, each substrate will carry its ownreference marks, and the above-described procedure will be followed withrespect to the reference marks of each substrate.

Preferably, the electronic images are transformed in the above-describedmanner in real time during the exposing operation, but they may also betransformed off-line before the exposing operation. An advantage of thenovel method is that it enables the file to be transformed on-line andin real time during the exposing operation, from one format (e.g., avector format, such as post-script) to a bit-map format (e.g.,rastering), which otherwise would be a time-consuming process. Thus, theformat transformation process can be carried out in parallel with theabove-described coordinate transformation process, and would therebysave time and increase throughput.

Another advantage in performing geometrical transformations during theimaging or other processing operations is that this allows for real-timecorrections and adjustments in the processing operation according todata gathered on-line at that time. For example, if a first processingoperation is performed on the substrate (e.g., hole-drilling), andmachine errors are introduced in that processing operation, this mayresult in registration problems with respect to a second processingoperation (e.g., imaging a conductor pattern) to be performed on thesame substrate and to have a critical relationship with respect to thefirst processing operation. Even though the conductor pattern file andthe drilling file are properly converted to machine coordinates (e.g.,as described above), nevertheless an error in the drilling machine whendrilling the holes could subsequently affect the critical relationshiprequired between such holes and the conductive patterns (e.g., conductorpads) to be produced in a subsequent imaging operation.

A similar registration problem may arise if other processing steps areapplied on the substrate between the first and second processes in whichthe substrate may experience dimensional and/or geometrical changes, ordue to temperature changes between the first and second processes. Inthe example of PCB inner-layer drilling and imaging, such intermediateprocessing steps may include resist-coating, copper etching, lamination,multi-layer casting, etc.

FIG. 13 is a flow chart illustrating a method particularly addressingthe foregoing registration problems. Thus, as shown in FIG. 13, thefirst processing operation is performed on the substrate (block 50); thesubstrate is mounted on the machine to perform the second processingoperation (block 51); and as the second processing operation progresses,preselected features on the substrate from the first processingoperation are sensed (block 52) and used to control the secondprocessing operation (blocks 53-55). The sequence of identifyingfeatures from the first process, comparing their positions with theiranticipated positions (e.g., their coordinates in a suitable filetransformed to machine coordinates), and controlling the secondoperation in accordance with deviations of these features from theiranticipated positions, is performed repeatedly during the secondoperation, until the second operation is accomplished in accordance withthe first operation (block 56). Being suitably controlled, the secondprocess follows the path of the features of the first process inreal-time, resulting in registration between related features of thefirst and second processes.

FIG. 14 illustrates a specific implementation of FIG. 13 wherein thefirst processing operation of block 50 is the drilling of holes; and thesecond processing operation of blocks 52-56 is plotting an image on anetch resist coating for producing conductive pathways including pads.

Thus, as shown in FIG. 14, the original drilling file is in substratecoordinates (block 60), and the original circuit file is also insubstrate coordinates (block 61). As shown by block 62, thesecoordinates may be transformed to machine coordinates in the electronicmanner described above for transforming the electronic image of eachfile, using the at least two reference marks applied to the substrate;alternatively, the substrate may merely be aligned to the machine usingother techniques, such as one of the prior art techniques for physicallyre-orienting the substrate to the machine coordinates after thesubstrate has been applied to the machine.

The drilling file is then executed based on the machine coordinates(block 63); and the circuit file is executed based on the machinecoordinates (block 64). The execution of the circuit file is controlledby selecting features (holes) on the substrate produced by the drillingfile, as shown by block 65, in accordance with the imaging processindicated by the large block 66.

This imaging process of block 66 thus includes snapping one or more ofthe preselected features, e.g., the holes, (block 66 a), extractingdeviations from the expected positions by comparing the actual, sensedpositions with the nominal positions (block 66 b), calculating the localgeometric transformations to correct for these deviations (block 66 c),and using the calculated local geometrical transformations fortransforming the electronic image from the circuit file (block 64)during the plotting of the image of the circuit file (block 66 e).Accordingly, the plotting of the image from the circuit file, andparticularly the image of the conductive pads of the circuit file, willbe assured of having the proper critical relationship required withrespect to the holes produced during the previous execution of thedrilling file.

FIG. 14 also more particularly shows the repeated loop of blocks 52-55in FIG. 13 after the first processing operation (drilling of holes) hasbeen performed, and the substrate has been loaded on the imager toperform the second processing operation (plotting the image on theetch-resist coating).

FIG. 15 is a flow chart corresponding to that of FIG. 14, butillustrating the process wherein the first processing operation is theplotting of the image on the etch-resist coating for producingconductive pathways including pads; and the second processing operationis the drilling of holes which is controlled by the pad features sensedafter the substrate has been loaded onto the drilling machine. Thus,blocks 70-75 correspond to blocks 60-65 in FIG. 14, except the firstprocessing operation is to execute the circuit file, and the secondprocessing operation is to execute the drilling file; and blocks 76 a-76e within large block 76 illustrate how the execution of the drillingfile is controlled by selecting features (e.g., pads) produced duringthe previous execution of the circuit file, to assure that the drilledholes will have the required critical relationship with respect to thepads.

Although it is preferred from throughput considerations that theidentification of features from the first process and the realtedcontrol of the second process be carried out in real-time, all thepreselected features from the first process can be identified first, andthen the file of the second process can be modified accordinglyoff-line. The second process would then be performed upon the modifiedfile. This option may be preferable, for example, where not all thedesired transformations can be performed in real-time.

It is also optional, in such case, that the machine which identifies thefeatures of the first process need not be the machine which performs thesecond process. In this case, the features would be identified prior toloading the substrate on the machine which performs the secondoperation.

In addition to avoiding registration problems because of errors causedduring a prior processing operation as described above with respect toFIGS. 13-15, the novel method also enables other changes to be made inreal-time (or off-line) with the performance of an imaging operation, orother processing operation. For example, as indicated earlier the pixalincremental positions along the drum axis may be changed to change thescaling along the drum axis. This may be done in real time by changingdata transfer rates to the substrate (pixal data stroke frequency)and/or by changing the drum (or flat bed) velocity. Such featuresprovide a number of advantages including: extreme accuracy; the abilityto address in a sub-micron level far beyond the pixal size; the abilityto make on-line changes in scaling by changing carriage speed; and theability to change plot start positions along the drum axis as plotprogresses, to make jumps in the positions in the memory buffer fromwhich data is taken for the strokes, and to achieve a relative shift ofthe image along the carriage axis between several imaged layers bytaking the data from various images and from different locations in thesame memory buffer.

Another type of scaling correction that may be made in real time withthe processing operation is one to compensate for geometrical changesdue to deviations in the thickness of the substrate. This is shownschematically in the diagram of FIG. 16, wherein it will be seen thatthe autofocus device 12 (or 112, FIG. 9), in automatically focussing thelasers 6 (or laser 106) on the surface of the substrate 2 (or 102) toreceive the printed image, also produces a measurement of the thicknessof the substrate. The autofocus thickness readings (block 80) areextracted and continuously sent to the machine processor 16 inreal-time. The processor translates the thickness deviations to scalingcorrections (block 81) in real time, and compensates accordingly in itscontrol of the printing lasers.

While the invention has been described with respect to preferredembodiments, it will be appreciated that these are set forth merely forpurpose of example, and that many other variations and applications ofthe invention may be made. For example, the invention could be appliedto working operations other than printing, e.g., punching, drilling orcutting workpieces. The invention could also be applied to internalrotary-drum plotters as well as external rotary-drum plotters, and alsoto X-ray plotters for sensing embedded features. In addition, instead ofspecially applying the sensible reference marks, reference points onfeatures applied to the workpiece (e.g., substrate) in accordance withthe respective working (e.g., printing) file could be used as thereference marks or features since their nominal (actual) locations arealready known in terms of the coordinance of the respective workpiecefrom the respective working file. In addition, various sensing devices(e.g., electrical, magnetic, mechanical, X-ray camera or other opticaldevices) could be used for sensing the reference marks or the referencefeatures.

Many other variations, modifications and applications of the inventionwill be apparent.

What is claimed is:
 1. A method of processing a substrate comprising thefollowing steps: (a) performing a first processing operation on thesubstrate; (b) mounting the substrate on a processing machine to performa second processing operation thereon; (c) sensing the actual locationsof preselected features on the substrate produced by said firstprocessing operation; (d) and controlling said second processingoperation in accordance with the sensed features; wherein said steps (c)and (d) are performed by: (i) determining, before step (b), the nominallocations of at least one reference feature on the substrate in terms ofthe coordinate of the substrate; (ii) after step (b), sensing, andmeasuring the location of, said reference feature in terms of thecoordinates of said processing machine to perform the second operationon the substrate; (iii) determining at least one geometricaltransformation needed to transform the substrate coordinates of thenominal location of said reference feature to the machine coordinates ofthe sensed location of said reference feature; and (iv) controlling saidsecond processing operation in accordance with said at least onegeometrical transformation.
 2. The method according to claim 1, whereinthe actual location of each of the workpiece reference marks isdetermined in step (c) by; fixing a sensing device to the processingmachine for sensing the workpiece reference marks, with a referencepoint of the sensing device being at a known location with respect to areference point of the machine; and vectorially adding the line vectorfrom the machine reference point to the sensing device reference point,and the line vector from the sensing device reference point to therespective workpiece reference point.
 3. The method according to claim1, wherein the actual location of said substrate reference mark ismeasured in step (ii) by: (1) fixing a camera to the processing machinesuch that a reference point of the camera field of view is at a knownlocation with respect to a reference point of the machine; (2) actuatingthe camera to view a portion of the substrate which includes thesubstrate reference feature; (3) measuring the location of a referencepoint on the substrate reference feature in the camera field of viewrelative to the camera field reference point; (4) and vectorially addingthe line vector from the machine reference point to the camera fieldreference point, and the line vector from the camera field referencepoint to the substrate reference point, to thereby determine thelocation of the respective substrate reference point relative to themachine reference point.
 4. The method according to claim 1, whereinsteps (ii)-(iv) are performed for each preselected substrate feature inreal-time during the second processing operation.
 5. The methodaccording to claim 1, wherein said step (iii) includes the determinationof scale transformation, rotation transformation, X-axis shifttransformation, and Y-axis shift transformation.
 6. The method accordingto claim 1, wherein said second processing operation is an imagingoperation for producing an image on said substrate by a plotting head ofan imaging machine.
 7. The method according to claim 6, wherein saidimaging machine includes a rotatable drum on which said substrate ismounted, and said plotting head scans the substrate mounted on the drumalong an axis parallel to the rotary axis of the drum.
 8. The methodaccording to claim 6, wherein said imaging machine includes a flat-bedfor mounting said substrate, and a drive for driving said flat-bed alongone orthogonal axis; and wherein said plotting head scans the substratealong the other orthogonal axis with respect to the substrate.
 9. Themethod according to claim 6, wherein said substrate is a layer of aprinted circuit board, one of said processing operations involves theformation of holes through the layer, and the other processing operationinvolves the formation of pads on the layer to have a predeterminedrelationship to said holes in the printed circuit board.
 10. The methodaccording to claim 9, wherein said first processing operation is theformation of said holes, which holes are used as said preselectedfeatures on the substrate, and said second processing operation is theformation of said pads.
 11. The method according to claim 9, whereinsaid first processing operation is the formation of said pads, whichpads are used as said preselected features on the substrate; and saidsecond processing operation is the formation of said holes.